1. Field of the Invention
The present invention relates generally to magnetic storage devices, more particularly to magnetic recording heads and, more specifically, to a device for reducing crossfeed in a magnetic tape, data storage device having a multi-head configuration.
2. Description of the Related Art
From the start, digital computers have required some form of data storage as an adjunct to their relatively sparse main memory facilities.
Magnetic tape devices have been found to be a fast, efficient, economical means for storing computer data, such as for backing-up hard disk software application programs and data created utilizing the programs or for off-line file management tasks routinely implemented between a disk and tape as data is processed.
Streaming magnetic tape drives, also called streamers, are constant speed transports for storing information from computer hard disk drives to provide a portable, backup storage of the computer memory. Such drives generally record bidirectionally, laying down as many parallel tracks as tape head technology will permit in what is commonly called serpentine recording; current technology is about twenty-six tracks on quarter-inch tape. A basic description of computer tape drive technology can be found in STREAMING, Copyright 1982, Archive Corporation, Library of Congress Catalog No. 82-072125.
Shown in FIG. 1A is a standard tape head block 25 having multiple heads 11 for serpentine recording of tracks 0 through 3 on recording media 13, in this case, tape segment 13. Such a head block 25 has five "heads" ("R" representing "read;" "W" representing "write;" and "E" representing "erase"). Generally, as shown in FIG. 2, each head 11 comprises individual magnetic cores 17 having air gaps 15 where magnetic flux emanates for writing or erasing and is sensed for reading. Thus, a head block 25 as in FIG. 1A would have five head elements 11. Each core 17 has windings 19, 19' for sensing the electromagnetically induced current when reading magnetic flux changes from a media 13 crossing the gap 15 or for generating the magnetic flux via the gap 15 when writing or erasing on the media 13.
As will be recognized by a person skilled in the art, the dimensions in these FIGURES are greatly exaggerated since an actual head may have a gap of only about two micrometers in a data recording head. Dimensions in head design will vary depending upon the intended function and use, e.g., read, or "playback," write, erase, audio, instrumentation, video, digital audio tape, analog magnetic recording tape, magnetic disks. Gap spacing in the head block also varies depending upon the use of the head, but in a five head block 11 such as depicted in FIGS. 1A and 1B it will generally be about 0.2 inch horizontally by 0.15 inch vertically.
During data verification process by a streaming tape drive, both write and read operations are performed simultaneously, also known as "read-after-write" (RAW) or "read-while-write." Furthermore, in some instances, the erase head is also activated simultaneously. Because of the close relative proximity to each other, a problem arises in that the heads will be loosely coupled through stray electromagnetic flux which emanate from each head as shown in FIG. 1B, represented at an adjacent head throughout the drawings by arrow B.sub.EXT. This coupling can cause a high level of interfering signal (write, erase or both), particularly in the read channel. This interference can be detrimental to the read operation such as where a head has been adjusted to recognize only those recorded signals having a particular minimum threshold. This phenomenon is commonly known as crossfeed or crosstalk. Unwanted crosstalk signals may also be coupled from adjacent tracks into a core, stray flux from recorded tracks on media or from transformer coupling between core windings.
The common method to combat crossfeed problems is to wind the read head core 17 in the manner as shown in FIG. 2. The read head winding consists of two halves 19, 19' wound in the same directions and coupled in series through a load 21. For the purpose of this disclosure, as used throughout including the claims, "same direction" should be interpreted as to mean identical when taken from a common aspect, such as from the head gap 15 as shown in FIG. 5; e.g., both wound counterclockwise when viewed by an observer in the gap; or, alternatively, when considered as if the core halves were a straight bar. Thus, magnetic flux, /B, in the read head core 17 will produce in both half windings 19, 19' a read signal of the same amplitude and polarity. However, external stray magnetic flux B.sub.EXT will induce in each half winding interference signals of opposite polarity that will add algebraically and compensate each other due to the series connection of the two half windings 19, 19'.
A second component of the crossfeed problem is capacitively coupled interference signals. Referring to FIG. 3, such a signal is represented by voltage source E.sub.INT, coupled through stray capacitances CS1 and CS2. For example, such capacitive coupling interference may be caused by signals at an adjacent head or adjacent cabling. Stray capacitance between winding leads and ground are represented by equivalent stray capacitances CS3 and CS4. Because, in general, the windings and winding leads will not be exactly the same distance from the interference source E.sub.INT, stray capacitances CS1 and CS2 will not be of equal value.
As illustrated in FIG. 3, a known method for reducing such capacitively coupled interfering signals is to use a differential amplifier 23 with a sufficient common mode rejection ratio. However, the degree of mutual compensation of the interference signals is strongly dependent on signal balance. Thus, if two interference signal levels in each winding are not approximately equal, the known methods for compensation become inadequate.
Thus, to have sufficient suppression of the crossfeed, it is essential to:
balance interference signals in both half windings 19, 19' of the read head in the case of magnetic coupling, and
balance interfering signals at the inverting (-) input and non-inverting (+) inputs of the head output differential amplifier 23 in the case of electrical coupling.
The problem is exacerbated in a streaming tape drive head as specified by the Quarter Inch Committee (QIC) standard promulgated for 1.3 Gigabyte, 30 track, recording, as depicted schematically in FIG. 4. Here the read head gap length is approximately 17 microinches with a center write head gap of approximately 75 microinches and an erase gap of 500 microinches in a new configuration which presents the need for a more sophisticated solution to the crossfeed problem.